This document has been prepared as the Guide for Geography teachers, so that the can have a clear concept about the topic

1. Determination of Soil Reaction (pH) :
Theoretical Concept (Teacher’s Guide)
UG Geography Honours CBCS Syllabus
Part of GEO-A-CC-4-10-P: Soil and Bio-Geography Lab:
30 marks, 2 Credits
Study Material: Part III
Dr. Sarada Mandal
Associate professor in Geography
Prabhu Jagatbandhu College

2. Definition
• pH is a figure expressing the acidity or alkalinity of a
solution on a logarithmic scale on which 7 is
neutral, lower values are more acid and higher
values more alkaline. The pH is equal to −log10 c,
where c is the hydrogen ion concentration in moles
per litre.
• In soils, it is measured in a slurry of soil mixed with
water (or a salt solution, such as 0.01 M CaCl2),
and normally falls between 3 and 10, with 7 being
neutral. Acid soils have a pH below 7 and alkaline
soils have a pH above 7. Ultra-acidic soils (pH < 3.5)
and very strongly alkaline soils (pH > 9) are rare.

3. Factors affecting soil pH
The pH of a natural soil depends on the
• 1. mineral composition of the parent material of the
soil, and
• 2. weathering reactions undergone by that parent
material.
• In warm, humid environments, soil acidification occurs
over time as the products of weathering are leached by
water moving laterally or downwards through the soil.
In dry climates, however, soil weathering and leaching
are less intense and soil pH is often neutral or alkaline.
• pH of a soil affects nutrient and water availability to the
plants.

4. Buffer solution
• A buffer solution (more precisely, pH buffer
or hydrogen ion buffer) is an aqueous
solution consisting of a mixture of a weak acid and
its conjugate base, or vice versa. Its pH changes very
little when a small amount of strong acid or base is
added to it. Buffer solutions are used as a means of
keeping pH at a nearly constant value in a wide
variety of chemical applications. In nature, there are
many systems that use buffering for pH regulation.
For example, the bicarbonate buffering system is
used to regulate the pH of blood.

5. Glass Electrode
• A glass electrode is a type of ion-
selective electrode made of a doped
glass membrane that is sensitive to a specific
ion. The most common application of ion-
selective glass electrodes is for the
measurement of pH. The pH electrode is an
example of a glass electrode that is sensitive
to hydrogen ions.

6. pH Meter
• A pH meter is a scientific instrument that measures
the hydrogen-ion activity in water-based solutions,
indicating its acidity or alkalinity expressed
as pH. The pH meter measures the difference
in electrical potential between a pH electrode and a
reference electrode, and so the pH meter is
sometimes referred to as a "potentiometric pH
meter". The difference in electrical potential relates
to the acidity or pH of the solution. The pH meter is
used in many applications ranging from laboratory
experimentation to quality control. It is more
accurate method than the pH strip.

7. A digital pH meter
• contains a pH probe that passes the electrical signals to
the pH meter and it displays the pH value of the
solution. The glass pH probe itself contains two
electrodes, a sensor electrode and a reference
electrode. These electrodes are in the form of glass
tubes, one contains pH 7 buffer and the other contains
potassium chloride solution. Probe is inserted into the
solution to be tested.
• pH meters are used for soil measurements
in agriculture, water quality for municipal
water supplies, swimming pools, environmental
remediation; brewing of wine or beer; manufacturing,
healthcare and clinical applications such as blood
chemistry; and many other applications.

8. Principle of operation
• The design of the electrodes is the key part: These are
rod-like structures usually made of glass, with a bulb
containing the sensor at the bottom. The glass
electrode for measuring the pH has a glass bulb
specifically designed to be selective to hydrogen-ion
concentration. On immersion in the solution to be
tested, hydrogen ions in the test solution exchange for
other positively charged ions on the glass bulb, creating
an electrochemical potential across the bulb. The
electronic amplifier detects the difference in electrical
potential between the two electrodes generated in the
measurement and converts the potential difference to
pH units.

9. The Reference Electrode
• It is insensitive to the pH of the solution, being composed of a
metallic conductor, which connects to the display. This
conductor is immersed in an electrolyte solution, typically
potassium chloride, which comes into contact with the test
solution through a porous ceramic membrane. The display
consists of a voltmeter, which displays voltage in units of pH.
• On immersion of the glass electrode and the reference
electrode in the test solution, an electrical circuit is
completed, in which there is a potential difference created
and detected by the voltmeter. The voltage varies from test
solution depending on the potential difference created by the
difference in hydrogen-ion concentrations on each side of the
glass membrane between the test solution and the solution
inside the glass electrode.

10. Electrode (continued)
• For simplicity, many pH meters use a combination
probe, constructed with the glass electrode and the
reference electrode contained within a single probe.
• When the probe is placed in a solution to measure pH,
hydrogen ions accumulate around the bulb, and replace
the metal ions from the bulb. This exchange of ions
generates some electric flow that is captured by the
silver wire. Increase in the acidity of the solution has a
greater concentration of hydrogen ion that increases
the voltage. This increased voltage decrease the pH
reading in the pH meter. In the same manner, an
increase in alkalinity leads to decrease in the hydrogen
ion concentration and increase in hydroxyl ion. It also
decreases the voltage and increases the pH value in the
pH meter.

11. Maintenance
• The pH meter is calibrated with solutions of
known pH, typically before each use, to
ensure accuracy of measurement. To measure
the pH of a solution, the electrodes are used as
probes, which are dipped into the test solutions
and held there sufficiently long for the hydrogen
ions in the test solution to equilibrate with
the ions on the surface of the bulb on the glass
electrode. This equilibration provides a stable pH
measurement.

12. Maintenance (Continued)
• Because of the sensitivity of the electrodes to
contaminants, cleanliness of the probes is essential
for accuracy and precision. Probes are generally kept
moist when not in use with a medium appropriate for
the particular probe, which is typically an aqueous
solution available from probe manufacturers. Probe
manufacturers provide instructions for cleaning and
maintaining their probe designs. For illustration, one
maker of laboratory-grade pH gives cleaning
instructions for specific contaminants: general cleaning
(15-minute soak in a solution of bleach and detergent),
salt (hydrochloric acid solution followed by sodium
hydroxide and water), grease (detergent or methanol),
clogged reference junction (KCl solution), protein
deposits (pepsin and HCl, 1% solution), and air bubbles.

13. Why should we test soil pH?
• Correct soil pH is essential to ensure optimal plant
growth and crop yield, because it allows nutrients
to be freely available for plants to take in. Testing
the pH of your soil helps to determine what plants
are best suited for that area.
• Sometimes soil needs supplements, like fertilizers
and soil pH adjusters, for plants to be able to thrive.
Measuring the pH can help you figure out what and
how much you need.

14. Things That Affect Soil pH
• Many things can affect the pH of your soil. The
most common factors are climate and
weather, other plants in the area, the pH of
your irrigation water, soil type, the kind
of fertilizer you use, and nutrient availability.

15. Climate and Weather
• Temperature, precipitation, sunlight, and seasonal
weather changes all influence the soil pH. High
precipitation, for example, will wash essential nutrients
out of the soil. Many nutrients, such as calcium
carbonates, are basic so as these nutrients leave the
soil, it becomes more acidic.
• Water coming into contact with decaying material in
the soil (like leaves, for example) can also cause the pH
to drop, because decaying matter releases carbon
dioxide. When carbon dioxide mixes with water, acids
can form.
• Drier climates or regions going through a drought will
have a more alkaline soil pH. Because there isn't as
much water moving through the soil, minerals and salts
become concentrated, increasing the pH.

16. Plants
Native plants and local ecology can
determine the starting pH of your soil.
Soil underneath grasses is usually less
acidic, while soils formed under trees
tend to be more acidic. This is due to
there being more decaying matter
(leaves) near trees. The very crop you are
growing can even alter your soil’s pH.

17. Irrigation Water
• The water that you use to irrigate
your crops will also influence the pH
of your soil. If the water used is more
acidic or more basic than the soil it is
irrigating, the pH of the soil will shift.

18. Soil Type
• Did the soil in your region form from granite,
limestone, or shale? These parent materials will
determine if your soil is more acidic or more basic.
Areas with a lot of shale tend to be more acidic,
while areas rich in limestone are more basic.
• The texture of your soil will also determine how
easy or how difficult it is to adjust the pH; this
is known as the soil’s buffering capacity. Sandy soils
have a lower buffering capacity while soils with
more clay will have a higher buffering capacity. It's
harder to change the pH of soils with higher
buffering capacities.

19. Fertilizer
• Fertilizing soil is very important to get the best crop
yields. Because pH will affect how easily available
nutrients are to plants, it's important to check soil
pH before and after adding any type of fertilizer. By
knowing your pH you can decide how much
and what type of fertilizer you need.
• Artificial nitrogen fertilizers tend to lower pH the
most in soils. Organic fertilizer will acidify the soil
once they come in contact with water, because of
the soluble organic acids they contain.

20. Nutrient Availability
• Plants cannot absorb nutrients if the soil pH is too low or
too high. When the nutrients become bound up, plants will
not be able to take in what they need to grow.
• Most nutrients are available when the soil is slightly acidic,
but different plants thrive in different pH ranges depending
on their specific nutrient requirements. If the pH is too low,
aluminum toxicity can occur. When this happens, aluminum
becomes unbound and the plants take it in at toxic levels.
• If the pH is too high, nutrients like iron become bound.
Without adequate iron uptake, plants will lose their
chlorophyll and start to turn yellow, indicating the plants
can no longer make food for itself. Molybdenum
poisoning can also occur in soils with alkaline pH, resulting
in stunted crops.

21. Optimal Soil pH Ranges
• Plants that thrive in more acidic soil include apple
trees (pH 5 – pH 6.5), potatoes (pH 4.5 – pH 6), and
orchids (pH 4.5 – pH 5.5). Alkaline loving plants
include acacia and walnut trees (they both like soil
between pH 6 – pH 8).
• To figure out the best pH for your needs, do a little
bit of research on the type of plants that you want
to grow. Natural soil is typically between pH 4 and
pH 8. If your soil's pH doesn't match the plants
optimal range, you'll need to treat your soil.

22. Types of Soil Treatments
• Soil too acidic? Popular options for treatment are
lime, calcium carbonate, and ground up
eggshells. If the soil is too basic then gypsum,
iron sulfate, sulfuric acid, or calcium chloride can
be added.
• Irrigating the soil frequently can help lower the
pH if it is too high as well. However, be careful
not to over-water the plants if the treatment of
the soil is in a planted area. This can cause
diseases to set in, and nutrients can get diluted or
washed away.

23. Comparative analysis of different
methods of pH testing
pH Meter
Pros
• Small size,
• Better accuracy,
• Easy to keep clean
Con
• Need to know how to care
for the device
• Larger investment,
• More technical
Image of Digital pH Meter

24. Comparative analysis of different methods
of pH testing
pH strip paper
Pros
• Easy to use
• Inexpensive
• Quick procedure
Con
• Hard to read due to stain of soil,
• Loss of precision
• Hidden costs due to frequent
purchase
Image of pH Strip Paper

25. Comparative analysis of different
methods of pH testing
Reagents
Pros
1. Easy to use,
Con
• 1. Multiple kits needed,
• 2. hard to read due to stain
of soil,
• 3. limited number of tests,
• 4. Chemicals are hard to
dispose of.
Image of Reagent Kit

26. Determination of pH by pH strip paper
• pH test strips (litmus paper) are paper strips that have been
saturated with pH-sensitive dyes. When exposed to a damp
substance, the strips will change color relative to that substance’s
pH. This color change corresponds to a color chart provided with
the test strips. This method for testing is quick, easy, and
inexpensive, but it does have some disadvantages.
• Soil is very dark in color, even when mixed with water. The muddy
color could stain the test strips and make them hard to read. Even
when a color change can be seen, it's subjective since colors can
look different depending on the lighting, as well as from person to
person. This leads to inconsistent and poor results.
• Tests strips will not give the most accurate test results either;
they only have a resolution of 0.5 pH units. This means the closest
your test results can get to your soil's true pH would be 0.5 pH +/-.
Being 0.5 pH units off means a greater cost to treat the soil. If the
soil treatment is not accurate you can have low crop yield and
dead plants.

27. pH chemical test kits
• pH chemical test kits are like test strips in that they are
easy to use, but also have several drawbacks. Using a
soil test kit involves adding your soil, distilled or DI
water, and some chemicals (these will be included
when you purchase a kit) to a tube. The chemicals, like
test strips, react with the pH levels in your sample to
create a color change. Also like test strips, the color
change of the test kits will be subjective, and readings
will vary between different people.
• pH test kits have lower resolution, generally between 1
or 0.5 pH points, and tend to test specific ranges of pH.
That means you need to buy many kits to test your
different types of soil, or when you are just
beginning and don't know your starting pH.

28. pH chemical test kits (Continued)
• The number of tests you perform with a
chemical test kit limited to the number of
reagents included. Regular kits include enough
chemicals for anywhere between 1 and 10
tests. Disposal of these chemicals
poses another issue; chemical compositions
differ from manufacturer to manufacturer,
and many cannot just be poured down the
drain or into the trash.

29. Reference
• https://en.wikipedia.org/wiki/PH_meter
• https://www.pharmaguideline.com/2015/08/
principle-and-working-of-pH-probes.html
• https://www.hannaservice.eu/ultimate-guide-
testing-soil-ph/